Omeprazole is a known gastric proton-pump inhibitor (Merck Index II, 6800 and references cited therein). Other related compounds, all having the substituted 2-(2-pyridylmethylsulfinyl) benzimidazole or imidazopyridine structure, have been disclosed (see for example: U.S. Pat. No. 4,689,333; 4,808,596; E.P.O. Application Nos. 0 174 726, 0 240 158, 0 268 956, 0 302 720; Jap. Application Nos. H2-49774 and S61-178919; and Chem. Pharm. Bull. 38, 2853 (1990)).
Synthetic preparations of omeprazole and related compounds typically involve multistep syntheses of which the last step is the oxidation of a sulfide, for example pyrmetazole, which has the formula IIa, to the corresponding sulfoxide, for example omeprazole, which has the formula Ia. ##STR1##
Suitable oxidizing agents previously disclosed in the art for preparing these types of compounds include peroxides, such as hydrogen peroxide, m-chloroperbenzoic acid (MCPBA), perbenzoic acid and peracetic acid (see E.P.O Application No. 0 240 158), hypohalite salts, such as sodium hypochlorite (see EP-0 268 956), iodosobenzene, 3-methyliodosobenzene (see Spanish Application No. ES540,147), and other well-known oxidizing agents (see U.S. Pat. No. 4,255,431). The most preferred oxidizing agent is usually m-chloroperbenzoic acid. Suitable solvents previously disclosed include dichloromethane, chloroform, benzene, toluene, methanol, ethanol and the like. The preferred solvent is usually dichloromethane, an enviromentally harmful halogenated solvent. All of the references noted above describe isolation of the sulfoxide by extraction into an organic solvent.
More recently E.P.O. 302 720 discloses oxidation of this type of sulfide which utilizes aqueous hydrogen peroxide in the presence of a vanadium compound which acts as a catalyst. This reference discloses that oxidation under these conditions results in higher yields of the desired sulfoxide, less N-oxide by-products and, in some cases, isolation of the product directly from the reaction mixture by filtration. Vanadium compounds are known to be highly toxic.
P. Brougham et al. (Synthesis, 1015 (1987)) have disclosed that magnesium monoperoxyphthalate (MMPP) is a useful oxidizing agent which may be substituted for MCPBA in a variety of oxidative synthetic applications. The main advantages which were noted are the lower cost and reduced hazard of MMPP relative to MCPBA. However, it has been disclosed that replacement of MCPBA with MMPP may provide products that are distinctly different than the products obtained when the oxidizing agent is MCPBA. (See for example: A.B. Holmes et al., Synthetic Letters, 47 (1991) and V. Meladinis et al., Z. Naturforsh Sect. B, Chem Sci. 44, 1453 (1989)). MMPP is described as being soluble in water and low-molecular-weight alcohols, but is negligibly soluble in chloroform. The use of toluene as a solvent or co-solvent in an MMPP oxidation has not been previously described.
It is the object of the instant invention to provide a novel process for the preparation of anti-ulcer agents having a methylsulfinyl bridge from the corresponding compounds having a methylthio bridge.
It is also the object of the instant invention to provide a process for the preparation of such anti-ulcer agents which utilizes an oxidizing agent which is economically improved over, substantially free from hazards associated with and free from toxic components associated with previously described oxidizing agents in analogous processes.
It is also the object of the instant invention to provide a process for the preparation of such anti-ulcer agents which utilizes a solvent system free from environmentally harmful halogenated solvents.
It is further the object of the instant invention to provide a process for the preparation of omeprazole wherein the crude omeprazole is isolated by filtration of the reaction mixture thereby eliminating the need of a more expensive and time consuming extractive isolation procedure.